Ammonia is a gas with a high solubility in water, which is often used in an aqueous solution. Ammonia (NH3) is used in several industrial applications, among others for the production of nitric acid, urea and other ammonia salts, such as nitrates, phosphates, and the like. Ammonia derivatives are widely used in agriculture. Around 80% of the ammonia production is used for the manufacturing of fertilizers.
Commonly, ammonia is produced by synthesis of nitrogen and hydrogen according to the following exothermic reaction (i.e. a reaction which releases heat):N2+3H2↔2NH3+ΔHwherein ΔH is heat released by the reaction.
Ammonia production usually starts from a feed gas, which provides a source of hydrogen, such as methane, for instance. Nitrogen is obtained from air. Details of the ammonia production process are known to those expert in the field, and some features of the plant and process will be recalled later on, for a better understanding of the new aspects of the systems disclosed herein and for a better appreciation of the various and beneficial effects thereof vis-à-vis the plants of the current art.
Broadly speaking, the various process steps which are performed to produce ammonia from air and feed gas, require several compression trains. As understood herein, the term “compression train” indicates a machine aggregate comprising at least a driver and one or more compressors driven by the driver, to process one or more gaseous fluids. A gaseous fluid or gas as understood herein is any compressible fluid.
More specifically, in the ammonia production plants of the current art, a first compression train is required to compress the feed gas, such as methane, and deliver compressed feed gas to a primary steam reformer and to a secondary steam reformer. A second compression train is provided to compress process air and deliver compressed process air to the secondary reformer. Raw syngas (synthetic gas) obtained from shift conversion is compressed by a third compression train. A further, fourth compression train is required to process a refrigerant fluid, which chills the ammonia produced from the syngas in an ammonia converter.
FIG. 1 illustrates a schematic of an ammonia production plant 1, with a compression train arrangement according to the current art. In operation, feed gas, for instance methane (CH4), is delivered through a feed gas compression train 3 to a primary catalytic steam reformer 5. The feed gas compression train 3 comprises a first driver 7 and a feed gas compression section 8. This latter can comprise a compressor 9.
Process steam is delivered at 11 to the primary catalytic steam reformer 5, wherein feed gas reacts with steam to generate carbon monoxide and hydrogen according to the reactionsCH4+H2O↔CO+3H2 CO+H2O↔CO2+H2 
The primary reformer 5 is fluidly coupled to a secondary steam reformer 15, which receives the reaction products from the primary reformer 5 in addition to process air from process air inlet line 17. The process air is compressed by a process air compression train 19.
The process air compression train 19 comprises a second driver 21, which can drive a process air compression section 22. This latter can include for instance a first process air compressor 23 and a second process air compressor 25 arranged in series. An intercooler 27 can be arranged between the delivery of the first process air compressor 23 and the second process air compressor 25.
In the secondary steam reformer 15 the unreacted CH4 from the primary catalytic steam reformer 5 is transformed into carbon monoxide (CO) and carbon dioxide (CO2) by combustion. The resulting gas mixture is raw syngas, which is delivered to a shift conversion unit 29.
In the shift conversion unit 29 the carbon monoxide is converted into carbon dioxide according to the following reactionCO+H2O↔CO2+H2 
The resulting gas mixture is delivered to a scrubber 31, where carbon dioxide is stripped and the resulting gas mixture is delivered to a methanation section 33. The residual carbon monoxide contained in the gas flow from the scrubber 31 is converted by hydrogenation in the methanation section 33, generating CH4 and H2O according to the reactionsCO+3H2↔CH4+H2OO2+4H2↔CH4+2H2O
The gas mixture thus obtained is fed through a drier 35 and the resulting pure syngas, containing mainly nitrogen and hydrogen, is compressed by a syngas compression section 34.
The syngas compression section 34 can comprise one or more compressors driven by a third driver. In the schematic of FIG. 1, the syngas compression section 34 comprises a first compressor 36 and a second compressor 37, driven by a third driver 39. The compressors 36, 37 and the third driver 39 form a syngas compression train 41. Intercoolers 42, 44 can be provided between compression sections and/or between compressors of the syngas compression section 34
The compressed syngas is delivered to an ammonia converter 43 to produce the desired end-product ammonia according to the ammonia synthesis reaction3H2+N2↔2NH3 
Ammonia from the ammonia converter 43 is chilled in a chiller 45. Chilling is achieved by means of a refrigeration cycle, which comprises a refrigerant compression train 48. The refrigerant compression train 48 comprises a refrigerant compression section 46 and a fourth driver 47. The refrigerant compression section 46 can include a first compressor 49 and a second compressor 51 and can comprise an intercooler 53 between them.
The chilled ammonia flows through a liquid/gas separator 55, where the gaseous ammonia is removed and recycled through the syngas compression train, as pictorially represented by a recycling line 57.
The above briefly described ammonia production system is complex and requires four compression trains, each embodying one of the four compression sections, namely: the feed gas compression section 8; the process air compression section 22; the syngas compression section 34; and the refrigerant compression section 46. Each compression section is provided with a respective driver, namely first driver 7, second driver 21, third driver 39 and fourth driver 47, to form four compression trains, namely the feed gas compression train 3, the process air compression train 19, the syngas compression train 41 and the refrigerant compression train 48.
The requirement of several compression trains makes the ammonia production plant complex and expensive. It would therefore be desirable to simplify the general arrangement of an ammonia production plant.